A knowledge of specific synaptic mechanisms is fundamental to understanding how the nervous system functions. The overall objective of this proposed study is the identification and characterization of elements of a simple nervous system which may serve as a model system in which to study serotonergic transmission. Serotonin is a likely candidate for the role of cardioinhibitory transmitter in the anthropod, Limulus polyphemus. Thus, the general characteristics of the arthropod nervous system will be exploited. The immediate objectives of the proposed study are: (1) to establish the cardioregulatory role of serotonin by biochemical and electrophysiological methods and (2) to biochemically characterize individual, physiologically identified cardioinhibitory and cardioexcitatory cells. Sensitive and specific radiochemical assays in microbiochemical procedures will be used to measure single cell levels of transmitter system entities. Neuroanatomical techniques will be used to corroborate the neural network of cardioregulation as suggested by the electrophysiologic studies.